# Photons, no mass and momentum

1. May 27, 2013

### lntz

Hello,

I have a question that i'd like to ask, but the wording is a little tricky, so I hope you can get the gist of what i'm trying to describe.

I was thinking about the thought experiment that is often used to describe relative motion - a light clock on a train passing through a station being observed from the platform. From there I had an idea that isn't directly related, but it came out of the train.

Imagine you have a train travelling along a straight piece of track and on board there is a photon emitter aiming perpendicular to the direction of travel. (This is where I don't think I explain it too well). Before the photons have been emitted, can they be said to exist? I ask this because i'm wondering whether they already have momentum in direction the train is travelling in when they are emitted. So, would an emitted photon leave the train perpendicular to the direction of travel, or at some angle caused by a component in the direction of trains motion?

I hope that makes sense and that this is in the correct part of the forum,

Lntz.

2. May 27, 2013

### Staff: Mentor

According to train observers, the path of the photon is perpendicular to the direction of the train's travel. But according to platform observers, the path is at an angle.

3. May 27, 2013

### lntz

I should have thought this one through! I really should have realised the path would be different for the two observers...

4. May 27, 2013

### Staff: Mentor

Right. The speed of light is invariant, but not the velocity (direction).

5. May 27, 2013

### Naty1

That depends on the nature of your source, not on it's relative motion. Suppose you have a light bulb 'on' in a closed box and you transmit by opening a door in the box. Then suppose you keep the light turned 'off' until you wish to transmit.